Interactive effects of cycloheximide and puromycin in altering brain polyribosomes and neutral and behavioural responses to electroshock in mice

Passive avoidance impairment in rats following cycloheximide injection into the amygdala

Cycloheximide (CHX:1, 10 or 20 microg) was injected via indwelling cannulas into various regions of the rat brain and its effects on passive avoidance training were studied. Rats with 10 or 20 microg of CHX injected into the amygdala immediately after the training footshock exhibited amnesia for the learning experience when tested after 24 h. In contrast, animals injected with 20 microg of CHX at a site either in the internal capsule only 2 mm above the amygdaloid injection site or in the frontal cortex showed no retention deficit when tested after 24 h. A quantitative examination of protein synthesis in brain halves 30 min after unilateral injection of 20 microg of CHX into the amygdala demonstrated that total protein synthesis was inhibited by less than 10%. Autoradiographic studies revealed that this inhibition resulted from a profound, highly localized inhibition of protein synthesis in areas immediately adjacent to the cannula. A comparison of the regional patterns of protein synthesis inhibition caused by injection of CHX into either the amygdala or internal capsule suggested that CHX might produce amnesia by virtue of its localized effect on the amygdala. Control experiments revealed that injection of 20 microg CHX into the amygdala had no effect on short-term retention, or short-term performance. Injection of 20 microg of CHX into the amygdala 12 h after the footshock had no effect on long-term retention. The observed impairment of retention was shown to be dose-dependent as injection of 1 microg of CHX into the amygdala was without effect. In addition, it was demonstrated that the CHX-induced amnesia did not result from induction of local seizure activity. These data show that localized injections of small amounts of CHX into the amygdala can produce deficient memory of a training experience even though total brain protein synthesis is only slightly inhibited.

Translocon closure to Ca2+ leak in proliferating vascular smooth muscle cells

Vascular smooth muscle cells have a proliferative phenotype that is important in vascular development, adaptation, and disease. Intracellular calcium handling is thought to play pivotal roles in determining the properties of these cells, and thus previously unrecognized mechanisms for transmembrane calcium movement are of potential interest. An unsolved question is the mechanism of constitutive (passive) calcium leak from the intracellular stores. Studies of other cell types have suggested that the translocon is a calcium leak pathway. Here we investigated the contribution of the translocon in proliferating vascular smooth muscle cells. Calcium leak into the cytoplasm was measured using fura-2, and protein synthesis was measured using radioactive methionine. Puromycin, emetine, and anisomycin are chemicals that inhibit protein synthesis, acting via the translocon; all three agents strongly inhibited protein synthesis in the smooth muscle cells within 1 h. Puromycin, which opens the translocon, evoked a transient increase in cytoplasmic calcium that was similar in amplitude to the calcium rise evoked by thapsigargin. The puromycin effect was abolished by thapsigargin. The treatment of cells for 1 h with emetine or anisomycin, which close the translocon, inhibited the calcium release evoked by puromycin but not the calcium release evoked by extracellular ATP, endothelin-1, or the calcium ionophore ionomycin. Thapsigargin-evoked calcium rises were slightly suppressed by emetine but unaffected by puromycin or anisomycin. The data suggest that the translocon has the capacity to act as a calcium leak pathway in the ribosomal endoplasmic reticulum but that it is normally closed and lacks relevance to physiological calcium leak mechanisms.

Dicer expression and localization in post-mitotic neurons

RNA-mediated gene silencing is recently emerged as a fundamental mechanism of regulation of gene expression in many organisms and tissues, with special emphasis with respect to the nervous system. With the aim to study the components of RNA silencing machinery, we have investigated the expression profile and localization of dicer protein RNase III endonuclease in cultures of post-mitotic neurons. Dicer catalyzes the processing of double-stranded RNAs (dsRNAs) into approximately 21-25 nucleotide-long small interfering (si)RNAs and micro (mi)RNAs, and it represents an essential step in the biogenesis of these small non-coding RNA molecules. We show that in rat primary neurons dicer is localized in the somatodendritic compartment, at the Golgi-reticulum area network level. This peculiar distribution was altered by brefeldin A treatment. Moreover the Golgi-reticulum dicer signal was observed also in primary astroglial cells. In addiction dicer was observed to be regulated during the embryogenesis and development in several tissues. In fact its expression is developmentally regulated in cultured cerebellar granule neurons. This is the first study in which dicer is shown preferentially distributed in the Golgi-reticulum area in post-mitotic terminally differentiated neuronal and glial cells and that its profile is modulated during maturation and development of in vitro cultured cerebellar granule neurons.

Ischemic thresholds of cerebral protein synthesis and energy state following middle cerebral artery occlusion in rat

The ischemic threshold of protein synthesis and energy state was determined 1, 6, and 12 h after middle cerebral artery (MCA) occlusion in rats. Local blood flow and amino acid incorporation were measured by double tracer autoradiography, and local ATP content by substrate-induced bioluminescence. The various images were evaluated at the striatal level in cerebral cortex by scanning with a microdensitometer with 75 microns resolution. Each 75 x 75 microns digitized image pixel was then converted into the appropriate units of either protein synthesis, ATP content, or blood flow. The ischemic threshold was defined as the flow rate at which 50% of pixels exhibited complete metabolic suppression. One hour after MCA occlusion, the threshold of protein synthesis was 55.3 +/- 12.0 ml 100 g-1 min-1 and that of energy failure was 18.5 +/- 9.8 ml 100 g-1 min-1. After 6 and 12 h of MCA occlusion, the threshold of protein synthesis did not change (52.0 +/- 9.6 and 56.0 +/- 6.5 ml 100 g-1 min-1, respectively) but the threshold of energy failure increased significantly at 12 h following MCA occlusion to 31.9 +/- 9.7 ml 100 g-1 min-1 (p less than 0.05 compared to 1 h ATP threshold value; all values are mean +/- SD). In focal cerebral ischemia, therefore, the threshold of energy failure gradually approached that of protein synthesis. Our results suggest that with increasing duration of ischemia, survival of brain tissue is determined by the high threshold of persisting inhibition of protein synthesis and not by the much lower one of acute energy failure.(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibition of protein synthesis during CNS myelination produces focal accumulations of membrane vesicles in oligodendrocytes

Optic nerves of Xenopus tadpoles were exposed to cycloheximide to identify changes that occur during CNS myelin membrane formation when protein synthesis is inhibited. Groups of stage 51-56 tadpoles were immersed in either 10 or 20 micrograms ml-1 cycloheximide, and at specified times between 12 and 18 h after initial immersion tadpoles were killed and their optic nerves prepared for ultrastructural analysis. As early as 12 h there were alterations in oligodendrocytes from treated animals compared with control animals. The number of polyribosomes in the perikarya and cell processes was greatly reduced and the rough endoplasmic reticulum was disorganized. Mitochondria and microtubules were normal in appearance. Many oligodendroglial tongue processes at the inner margin of the myelin sheath were enlarged, occasionally indented the axon and were filled with vesicular profiles. Vesicles were noted in other cytoplasmic regions of oligodendrocytes and focal changes in the lamellar structure of myelin were found in paranodal regions. The internodal portions of the myelin sheath, axons and astrocytes appeared normal. Polyacrylamide gels of optic nerves showed that the incorporation of 35S-methionine into polypeptides had been almost completely inhibited by treatment with cycloheximide. These observations suggest that cycloheximide, by inhibiting synthesis of myelin proteins, alters the ability of oligodendrocytes to incorporate membrane components into CNS myelin sheaths.

Cerebral protein synthesis during long-term recovery from severe hypoglycemia

Regional protein synthesis was investigated in the rat brain during long-term recovery from insulin-induced hypoglycemia with 30 min of cerebral electrical silence. At various time intervals up to 14 days after glucose replenishment, animals received a single dose of L-[3,5-3H]tyrosine and were killed 30 min later. Brains were processed for autoradiography using the stripping film technique. Although hypoglycemia sufficiently severe to cause cessation of EEG activity leads to almost complete inhibition of amino acid incorporation in all "vulnerable" forebrain structures (cerebral cortex, hippocampus, caudoputamen), autoradiographs revealed a very specialized sequence with differential posthypoglycemic restoration of biosynthetic activity in certain neuronal cell types. Three major subpopulations could be distinguished: Neurons that fully regained their protein synthetic capacity within 6 h following hypoglycemia (cortical neurons of layer III-VI, large neurons in the caudoputamen, CA3 and CA4 pyramidal neurons, the majority of granule cells of the dentate gyrus) seemed to escape neuronal necrosis. Prolonged impairment of protein synthesis with only partial restoration during the early posthypoglycemic recovery period (CA1 neurons, most small- to medium-sized neurons of the caudoputamen) carried an increased risk of permanent cell damage. The large majority of these neurons, however, showed full recovery of protein synthesis as late as 7 days after hypoglycemia. Neurons with complete lack of amino acid incorporation after 6 h of recovery (granule cells at the crest of the dentate gyrus, small neurons of the dorsolateral caudoputamen) never resumed protein synthesis, regressed, and died. These studies in conjunction with morphological analysis indicate that the sequential recovery of protein synthesis reflects the extent to which neuronal populations are at risk during severe hypoglycemia.

[3H]lysine incorporation into protein of hemisected, cycloheximide-treated spinal cord

A single application of a protein synthesis inhibitor, puromycin, in the lesion site of a spinal hemisection transiently decreases protein incorporation for less than 24 h and results in axonal sprouting and dendritic and synaptic stability for at least 60 days. In this study we characterized the protein-altering properties of single applications of a different protein synthesis inhibitor, cycloheximide, for comparison. The protein incorporation of [3H] lysine into hemisected rat spinal cord (left side, T2) treated with cycloheximide (100 micrograms/ml in Gelfoam sponge placed in wound at time of lesion) was tested 3 h, 6 h, 12 h, 1, 3, 7, and 14 days later and compared with that in 0.9% saline in Gelfoam implants as normal controls (N = 54). One hour prior to utilization, the animals were injected subcutaneously with 200 mu Ci L-[4,5(n)-3H]lysine monohydrochloride. No treatment-related differences in brain radioactivity were noted at any time postoperation. In spinal cord, inhibition of amino acid uptake by cycloheximide was not demonstrable at 3 h after hemisection, but could be detected from 6 h to 1 day after implantation. During that time, the TCA-soluble radioactivity at the lesion site was greater than in control animals (P less than 0.05). These results suggest that as with puromycin, morphologic effects produced by cycloheximide on spinal cord regeneration may correspond to chemical events which occurred within the first 24 h after cord hemisection and cycloheximide treatment.